Primary plant cell walls are fibrous composite structures consisting of cellulose microfibrils embedded in a heterogeneous polymer matrix of which a major constituent is pectin. In addition to a directly structural role, the pectic network provides a dynamic operating environment within the primary cell wall matrix and is involved in cell adhesion, regulation of cell wall porosity, cell wall extensibility and ionic status.
Pectins are immensely complex not only in their composition but also in their linkages and in their intermolecular bonds. In most cell types they principally comprise the following three types of polysaccharides: homogalacturonan (HG), rhamnogalacturonan I (RGI) and rhamnogalacturonan II (RGI).
The organization, integration and precise structures and functions of these pectic polysaccharides within primary cell walls is still not fully understood.
However, it is known that HG can contain long interrupted sequences of contiguous (.alpha.-1,4-linked-GalA residues (known as HG blocks) that may be interspersed with rhamnose residues either occasionally or as repeating GalA-Rha structures. In vivo, HG is thought to be synthesized in a form with extensive methyl-esterification at the C-6 carboxyl position.
It is also thought that the controlled de-esterification of pectin by pectin methyl esterases within the cell wall influences pectic gel formation, principally by means of the formation of calcium bridges between stretches of de-esterified HG blocks at `junction zones`. Furthermore, de-esterification also influences the susceptibility of HG to hydrolytic and degradative enzymes such as polygalacturonases.
The mechanical properties of pectin gels are important both structurally, in that they contribute to the resisting of turgor pressure, and physiologically in that they determine the porosity of the cell wall. It is thought that cell wall porosity may regulate, by physical exclusion, the access of cell wall modifying enzymes to polymer substrates and so influence changes in cell wall architecture. HG is also an important source of biologically active oligogalacturonides (OGAs) that appear to have roles as signalling molecules in both plant development and defence.
Thus, HG is a multi-functional pectic polysaccharide of primary cell walls involved in calcium cross-linking and gel formation, the regulation of the ionic status and porosity of the primary cell wall matrix and is a source of oligosaccharins functioning in plant development and defence.
In addition to the above commentary, it is known that pectin comprises highly branched regions with an almost alternating rhamno-galacturonan chain. These highly branched regions may also contain other sugar units (such as D-galactose, L-arabinose and xylose) attached by glycosidic linkages to the C3 or C4 atoms of the rhamnose units or the C2 or C3 atoms of the galacturonic acid units. The long chains of (.alpha.-1-4 linked galacturonic acid residues are commonly referred to as "smooth" regions, whereas the highly branched regions are commonly referred to as the "hairy regions".
As indicated above, some of the carboxyl groups of the galacturonic residues are esterified (e.g. the carboxyl groups are methylated). Typically esterification of the carboxyl groups occurs after polymerization of the galacturonic acid residues. However, it is extremely rare for all of the carboxyl groups to be esterified (e.g. methylated).
Usually, the degree of esterification ("DE") will vary from 0-90%. If 50% or more of the carboxyl groups are esterified then the resultant pectin is referred to as a "high ester pectin" ("HE pectin" for short) or a "high methoxyl pectin". If less than 50% of the carboxyl groups are esterified then the resultant pectin is referred to as a "low ester pectin" ("LE pectin" for short) or a "low methoxyl pectin". If 50% of the carboxyl groups are esterified then the resultant pectin is referred to as a "medium ester pectin" ("ME pectin" for short) or a "medium methoxyl pectin". If the pectin does not contain any--or only a few--esterified groups it is usually referred to as pectic acid.
A Protocol for determining the degree of esterification of the PME substrate may be found on page 58 of WO-A-97/03574. For ease of reference, this Protocol is recited below.